From patent CH-27961 there is known a timepiece including backward moving display means which permit reading of hours and minutes on circular arcs provided on a dial by means of two hands returning to their point of origin after having attained the respective divisions 12 and 60. Such timepiece responds to the definition given hereinabove.
Such piece includes a first driving intermediate wheel formed by a gear which is controlled by an entirely mechanical horometric movement and which meshes with a toothed sector, itself in mesh with a wheel fixed to a pipe bearing a minutes hand. The hours hand is borne by another pipe which in this case is directly arranged on a sector meshing with a second intermediate wheel.
Each intermediate wheel includes in its thickness a cut-out formed by a notch in the form of a thumbnail. Such notch opens out radially at the periphery of the intermediate wheel near the teeth thereof intended to mesh with the sector.
Thus, after each complete rotation in a first sense referred to as driven sense, the cutout appears facing the sector which is thus liberated from the intermediate wheel and which, thanks to an elastic return means, effects an automatic displacement in a rotation sense opposite to the first. Furthermore, the sector includes an abutment while the intermediate wheel includes a counter-abutment, such abutment and such counter-abutment being formed so as to come into contact in order to limit the angular displacement of the sector and to readjust precisely the corresponding hand onto its original position relative to the dial. The abutment which is provided on the sector is formed by a heel the height of which is twice that of the teeth and thus to that of the body of the sector. The counter-abutment is formed by a tooth of the intermediate wheel which is located above the cutout at a level superior to the gear teeth of the sector.
It is thus understood that the intermediate wheel must exhibit a sufficient thickness in order to be able to accommodate the cutout arranged therein and in order to allow a tooth forming a counter-abutment to remain which is sufficiently rigid in shear to accept the dynamic effort provoked by the contact of the abutment formed by the heel at the end of the displacement in the reverse sense.
It is also understood that the heel itself must exhibit a sufficient thickness or project sufficiently from the gear teeth of the sector in order to come into cooperation with the counter-abutment.
Thus, this arrangement exhibits the primary difficulty of being relatively cumbersome in thickness. Such arrangement had been conceived for application to a timepiece movement such as a large clock and it is the reason for which the designers of that time had not been particularly preoccupied with diminishing the thickness thereof. This arrangement thus may not be incorporated into a modern mechanical and/or electromechanical timepiece in which, as is known, it is furthermore necessary to provide space for other large volume components such as the battery or additional mechanisms for indication of the date, indeed of a chronograph.
Furthermore, this prior arrangement requires machining of the intermediate wheel for effecting the cutout in form of a thumbnail. This special operation as well as that for forming the heel are expensive and increase substantially the price of the timepiece so equipped.
Furthermore, there is known, according to patent CH-143 441, a timepiece one hand of which, in particular that of the hours, is also mounted fixed to a toothed sector which is intended to cooperate with an intermediate wheel in which is arranged a cutout also formed by a notch in the form of a thumbnail directly machined in the intermediate wheel. In this arrangement the abutment and counter-abutment are constituted respectively by a beam of substantial length and by an axis which are respectively mounted on the toothed sector and on the intermediate wheel. The axis is arranged in a tangential fashion to the cutout.
There also, the abutment and the counter-abutment project axially from the sector and from the intermediate wheel and in thickness occupy a substantial amount of space.
Finally, from patent CH-61 478 there is known a date display watch, one hand of which is mounted on a date wheel from which a certain number of teeth are lacking, such wheel being driven in rotation through a pin mounted on an intermediate wheel.
Thus, when the hand has finished its course in the sense of the driven displacement, the pin is located within the cut-away portion formed by the absence of teeth on the date wheel and the motion of such wheel as well as that of the hand are stopped in the absence of meshing with the intermediate wheel. Additionally, the date wheel meshes with a manually operated wheel on which is arranged a bare sector, the diameter of which is substantially identical to the diameter at the tips of the teeth of the manually operated wheel. This latter permits, in view of its bare non-toothed portion, to form an abutment limiting the angular displacement of the date wheel.
Thus, this arrangement is not automatic and necessitates the use of an additional operating wheel which, there as well, increases the costs of manufacture. Furthermore, such wheel does not permit assuring sealing and exposes the movement to dust and impurities since it projects from the case in order to be accessible from the exterior. One may also observe that the intermediate wheel must show a substantial reduction ratio since it must effect one revolution for each step of the date hand, this because of the driving pin.
Thus, the present invention has as its purpose to overcome the difficulties of the prior art mentioned hereinabove in furnishing a timepiece provided with display means having an entirely automatic backward motion and of the least possible thickness in order to diminish the height, in particular, so as to be adapted to equip standard movements of a predetermined thickness.
The present invention has also as purpose to furnish a timepiece of this type, the cost of manufacture and assembly of which are as low as possible.